A recording material using an electron-donating colorless dye and an electron-accepting compound is well known as a pressure-sensitive paper, a heat-sensitive paper, a light-sensitive pressure-sensitive paper, an electro thermo-recording paper, etc., as described, e.g., in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Publication No. 23922/85, Japanese Patent Application (OPI) Nos. 179836/82, 123556/85, and 123557/85 (the term "OPI" as used herein indicates an "unexamined published Japanese patent application").
These recording materials must have such performance as (1) giving sufficient color density and color sensitivity, (2) forming less or no fog, (3) giving colored material having sufficient fastness, (5) giving high S/N ratio, and (6) giving colored material having sufficiently high chemical resistance but recording materials completely satisfying these requirements have not yet been obtained at present.
In particular, a heat-sensitive recording material generally has a fault of forming fog by the action of solvent, etc., and a fault of causing discoloring or fading of colored materials by the action of fats and oils, chemicals, etc. Thus, when a heat-sensitive recording material is brought into contact with a stationery or office supplies such as a water-color ink pen, an oil ink pen, a fluorescent ink pen, a vermilion inkpad, adhesives, a diazo developer, etc., or cosmetics such as a hand cream, a milky lotion, etc., white portions (background portions) are colored or colored portions are discolored or faded to greatly reduce the commercial value thereof.
Accordingly, the inventors have investigated for obtaining good elements for recording materials and good recording materials by paying attention to the characteristics of electron-donating colorless dyes and electron-accepting compounds, such as the oil solubility, solubility in water, distribution coefficient, pKa, the polarity of substituent, the position of substituent, the crystallinity of a mixture thereof, the change of solubility, etc. Also, recently, the development of a recording material giving colored portions having absorption at infrared regions has been desired.